Introduction:

By Easter 2001 BUSCA, the 64 Megapixel Bonn University Simultaneous
Camera, will undergo the official commissioning procedure at Calar Alto.
From then on it will be available for the community. In April 2000 the
camera had its first Calar Alto test run. A second run followed in September
2000.

The number of CCD-pixels and the target telescope imply a certain
similarity with the WFI (at the 2.2m MPIA/ESO telescope) but BUSCA is based
on a completely different concept. The instrument is designed for
simultaneous four color photometry in a 12arcmin FOV. The light is split
into four wavelength bands from UV to visual IR using dichroic filters.
In the four corresponding focal planes the same area of the sky is imaged
onto 4Kx4K 15micron pixel CCD sensors. Additional filters are provided
to define standard wavelength bands. Currently the Strömgren filter
set is available.

This article gives an overview of the main instrument features and presents
some first results of our observing campaigns. A more detailed description
of BUSCA is given by Reif et al. (1999, have a look at the on-line
version ) and Reif et al. (2000) .

Instrument overview:

The photograph below shows BUSCA with its four CCD systems mounted at the
mirror cell of the 2.2m. The flat hexagonal section immediately below the
telescope flange houses the TV-guider system. Inside the cube are the shutter
the dichroic beamsplitter (K. Bagschik, 2001 [KB]) and filter wheels (4
wheels, one in front of each CCD dewar). Each filter wheel has four filter
positions. Two electronic boxes - the CCD controller and the instrument
control unit (Universal Control) - are attached.

The BUSCA TV-guider:

BUSCA has its own TV-guider which was constructed as part of the instrument.
All electronic components were provided by the MPIA and are compatible
with those of the original unit. The observer uses the standard TV-guider
user interface. The BUSCA TV-guider unit had to be much flatter than the
original one. As a consequence of the compact design, the guider field
is fixed and can not be moved with respect to the observed field. But with
a guider FOV of about 6arcmin x 4arcmin one can always find guide stars.
The large guider field of view is achieved by converting the aperture ratio
to f/4 by means of an additional achromatic lens. The position far outside
the optical axis results in a significant astigmatism. This is compensated
to a large degree by decentering the lens and by introducing two inclined
plane-parallel plates into the TV-guider beam [KB].

(If the standard 2.2m TV-guider unit were used the BUSCA image plane
would be to far behind the optimum focus for the 2.2m telescope. The height
of the original TV-guider unit and the long optical path through the BUSCA
beam splitter unit together produce a to large back focal distance.)

Wavebands and Filters:

The BUSCA wavebands have been defined [KB] with the two
most widely used non-overlapping filter systems in mind - Strömgren
(mainly used for stellar work) and Thuan-Gunn (for extragalactic work)
- and a separate visual NIR band. A complete Strömgren u,v,b,y set
is available. Two Strömgren Hbeta filters have been orderd and will
become available soon. For the visual NIR band a Bessell I filter was procured.
All color channels have one white glas (WG280) filter which allows to use
the full bandwidth.
The optical path lengths through simultaneously used filters
have to be identical. The CCD's can not be focussed individually.

The Four BUSCA wavebands. They are named - from left to right - uv,
b, r, nir. Indicated are positons and widths of standard line, intermediate
and broadband filters. Filters marked by white horizontal bars (u,v,b,y,
Bessell I) are at present available.

Available filters:
Effective tansmission of the available Strömgren and broad band
filters (CCD QE not included).

The BUSCA Shutter:

A typical problem of CCD cameras with large format detectors is to achieve
a uniform exposure across the full image. This problem is most severe
at short exposure times which are needed in case of - usually - bright
calibration sources or in order to extend the observing time for sky flats
at dusk/dawn. We have developed a shutter with the following main characteristics:

The shutter is a removable drawer-like unit. The flat part is inside
the instrument housing (the cube) and the left hand side box (with motors/encoders)
outside.

Shutter interiors: Visible are the two carbon fiber blades, the linear
ball bearing rails and the toothed belts.

In order to test the exposure unifomity flat fields were taken with
short (0.1sec) and long (10sec) exposure times. The comparison shows that
the remaining non-uniformities are much less than 1% across the full 4K
CCD along the direction of the shutter movement. This is consistent with
independent timing measurements: We find that the scatter in exposure time
across the shutter aperture is less than 1msec peak to peak.

CCDs:

BUSCA is equipped with individual CCD detector systems for each of the
four distinct wavelength bands. We have been looking for CCD solutions
that cover the entire 60mm x 60mm field of view with sufficient spatial
resolution, and with at least one thinned CCD for the uv channel, considering
various choices (2K x 2K or 2K x 4K mosaics and 4K x 4K monolithic detectors)
(R. Kohley, 1998). Budget constraints and the limitation set by the CCD
controller design (max. 4 CCD outputs) led us to choose monolithic 4K x
4K, 15mue, LMFS CCD485's of which one was thinned and coated by Mike Lesser.
(The thinned device was delivered by the end of 2000.)
The thick devices have a peak QE of about 45% at 600nm. The
thinned device has >70% at 350nm and >90% at 600nm. (Replacement of the
thick devices by the much more sensitive thinned ones is mainly a question
of getting it funded.)

Optical Performance
(at sub-arcsec seeing conditions):

The dichroic plane parallel plates of the beam splitter intersect the f/8
beam of the telescope. This produces an unacceptable large astigmatism.
With a simple "trick" this effect can be almost completely compensated
[KB]: With a second inclined plate of the proper orientation
the additional (orthogonal) astigmatism sums up with the primary one to
a defocus which is easily compensated. Although this has been tested and
confirmed on the optical bench (J. Schmoll, 1997): the real proof had to
be furnished at the telescope.
Luckily we had several hours with extraordinary good - sub-arcsecond
- seeing during both campaigns (in 2000). The PSF found in those BUSCA
frames demonstrates the good imaging performance (see the Jupiter image
below compared with a space based image).

BUSCA at the 2.2m vs. CASSINI:

The BUSCA image of Jupiter (left) was taken Sept. 10, 2000 in the BUSCA-uv
band (lambda < 430nm). The exposure ran while thin clouds covered unexpectedly
the Calar Alto sky. Minutes before, a clear sky
seeing of 0.8arcsec was measured. The CASSINI
image is from Oct. 8, 2000 (blue filter, centered at 445nm). Thanks
to a chance coincidence both exposures show the same rotational phase which
allows a detailed comparison. Many features and structures shown in the
CASSINI picture can easily be recognised in the BUSCA image.

To take advantage of those perfect seeing conditions the CCD's have
to be read out unbinned with full resolution (0.176arcsec per pixel). The
image of the globular cluster M13 is another example. Stellar images in
the four corners show consistently PSF's with a FWHM of 0.81arcsec.

M13 observed through the y-filter (exposure time: 5min).

Radial PSF profile of stellar images in the four outer corners of the
M13 frame. One pixel corresponds to 0.176arcsec. The FWHM's are 0.81arcsec
(4.6 pixel).

The radial PSF diagrams above show some residual scatter which is due
a weak PSF ellipticity (0.08). This ellipticity was found all over the
image and is mainly east-west oriented. After some investigation it was
realized that it is most probably caused by a small east-west oscillation
of the telescope structure. This is a known effect with a pretty small
amplitude of about 0.5arcsec which becomes detectable only in really good
seeing conditions.

The Observer's Work Place:

The instrument control is completely integrated into the Calar Alto environment.
For instrument communication and data acquisition the available hardware
(interfaces and cables) is used. The software for CCD camera and instrument
control was developed in Bonn and runs now on the "ultra3", the instrument
computer of the 2.2m. The observer has access through a graphic user interface
(tcl/tk based). One single window contains the basic elements for the control
of the CCD camera system and the filter wheels. One of the control features
of the software is the "real time display": While data are arriving during
data acquisition they are displayed in four thumbnail windows. For each
of the four CCDs individual FITS files are stored on disk.

The observers environment: On the lower left hand monitor the Busca
GUI with its characteristic 4 thumbnail windows for real time data display
(active during data acquisition).

Simultaneous Photometry of the Globular Cluster M71:

The observing program during the second run in September 2000 included
a sample of galatic globular clusters. They were observed through the v,b,y
filters. Here we present preliminary b,y-results for the globular cluster
M71. Two 15min frames were taken. A standard reduction (bias subtraction,
flat fielding) was performed followed by the DAOPHOT PSF photometry for
the coadded frames. The calibration is based on Strömgren standard
star data of Perry et al. (1987) (see diagrams below).

Magnitude and color calibration relations. Residuals with respect to
a slope of 1.0 show a scatter of 0.010mag for y and 0.012mag for
b-y.

Based on this calibration we derive the CMD shown below. To reduce the
influence of the field stars around M71 only stars within a distance of
2.5arcmin around the cluster center were included.

CMD of the low latitude globular cluster M71 (l = 56 deg, b = -5 deg,
E(B-V) = 0.7, varying over the cluster). Exposure time was 30 minutes.
The horizontal branch as well as the turn over point are well defined.
The diagram is not yet corrected for the influence of field stars.

From this diagram a limiting y-magnitude of about 20 mag can be derived
(exposure time: 30min, photometric error: 0.05mag). The CMD shows a well
defined red horizontal branch and turn off point. Some of the scatter in
the diagram may be due to differential reddening, although this has to
be investigated in detail. The red giant branch is not well populated as
already mentioned by Geffert et al. (2000).

Including the "v" frames in our analysis we will investigate metallicities
of red giants. Based on the m1 vs. b-y diagram metallicities will be derived
using the relation discussed by Hilker (2000). Similar studies of M22 and
M55 by Richter et al. (1999) show a large scatter in metallicity which
can not be attributed to photometric errors only. CN variations are currently
discussed as a possible explanation.

Summary and Outlook:

BUSCA has been developed with an efficient, reliable and easy to use instrument
in mind. Independent observers will tell us if this goal was achieved (we
are pretty confident).

After commissioning BUSCA will enter the pool of available instruments
at Calar Alto. As the Strömgren filter set is currently the only complete
set, it is to be expected that BUSCA will mainly be used for stellar work.
This may change in future, as new filters become available. The use of
existing filters from the observatory (if to be used simultaneously!) is
limited to those with identical optical path lengths. It should also be
kept in mind, that the 4K x 4K CCDs need large filters (diam.: 109mm) to
avoid vignetting. But if a reduced field of view is acceptable, smaller
filters could be inserted (adapter needed). A typical 2 inch filter reduces
the FOV to about 6 arcmin.

The limiting magnitude in the blue, red and NIR channels can be improved
by about 1 mag by replacing the thick frontside CCD's by backside thinned
detectors. It should also be mentioned that the thick CCD's have a lower
than standard CTE (charge transfer efficiency). This would also be improved
by new detectors.

Observing time can be used more efficiently if the 4-channel CCD-controller
were replaced by a faster multichannel system. This is particularly true
if under good seeing conditions the corresponding imaging quality can only
be achieved with an unbinned full resolution CCD read out (which
takes more than 6 minutes!). When the BUSCA system was designed the 4-channel
MPIA controller was the only choice. With BUSCA this controller is used
at its limits.

Acknowledgements:

The BUSCA project was realized with financial support by the Bundesminister
für Bildung, Wissenschaft, Forschung und Technologie through grant
05 3BN114 (4). We kindly acknowledge support and advice by B. Grimm of
the Max-Planck-Institute for Astronomy in Heidelberg.
We would like to thank the Calar Alto staff for their
help in getting the new instrument at the telescope and integrating it
in the Calar Alto environment.